During the maturation of T cells, expression of the CD4 and CD8 receptors responds to a complex mechanism of regulation; differential expression of the CD4 and CD8 glycoproteins is coupled to the choice of one of the pathways of differentiation either into helper T lymphocytes or into cytotoxic T lymphocytes; thus, thymocytes, that is the hematopoletic cells which are involved in a T differentiation pathway, first of all possess a CD4- CD8- phenotype (double negative or DN thymocytes); they then acquire joint expression of the CD4 molecule and the CD8 molecule, thereby forming double positive (DP) CD4+ CD8+ thymocytes and, finally, this population differentiates into single positive (SP) lymphocytes which express ether CD4, in the case of the helper T lymphocytes, or CD8, in the case of the cytotoxic T lymphocytes. Thymocytes which bind to class I histocompatibility molecules will become CD4+ CD8+ cytotoxic T lymphocytes while those which bind to class II molecules will become CD4+ CD8- helper T lymphocytes; after this intrathymic process of repertoire selection, the thyrnocytes leave the thymus and reach the peripheral system: i.e. blood and lymphoid organs.
This differentiation process is summarized in Nicolic-Zugic, J, (1991) Immuncol. Today 12: 65-70.
Many groups are currently studying the regulation of the expression of the CD4 gene since elucidation of the mechanism of this expression could contribute towards understanding the manner in which T cell development is controlled.
Several groups are working on the regulatory sequences of the human or murine CD4 gene. Those most recent studies which may be cited are the following: Killeen et al. (EMBO Journal Vol. 12 No. 4 p. 1547, 1993) demonstrated that a transgene carrying human CD4 of a size of approximately 35 kb possessed all the requisite genomic sequences for controlling expression during development in transgenic mice. Two distinct regulatory elements in this fragment were identified as being critical for expressing the transgene: the first is an enhancer sequence which is situated either 13 kb upstream of the cap site of the murine CD4 or 6 kb upstream of the human CD4. Blum M. D. et al. (J. Exp. Med. (1993) 177 No. 5: 1343-1358) identified and sequenced the human CD4 enhancer while Sawada et al. (Mol. Cell. Biol; 11 55: 5506-5515, 1991) identified and sequenced the murine CD4 enhancer. These two groups demonstrated that expression of this enhancer is specific for the CD4 gene in mature or immature T lymphocytes; P. Salmon et al. (Proc. Natl. Acad. Sci., USA 90: 7739-7743 (1993)) analyzed the structure and the sequence of the human CD4 promoter and compared it with that of the murine CD4 promoter. They identified a fragment of approximately 1100 base pairs which exhibits the function of a specific CD4 promoter. Aligning this sequence with that of the murine CD4 promoter indicates a very similar structure, as FIG. 1 of this latter paper shows.
Differentiation of the T lymphocytes employs other regulatory elements of the silencer type, whose presence leads to a decrease in, or cessation of, transcription of genes when they are in their vicinity. In Cell 77: 911-929, (1994), Sawada et al. demonstrated the existence of a silencer in an initial intron of the CD4 gene, one of the functions of which silencer is to extinguish transcription of the CD4 gene in mature CD8+ T lymphocytes.
Finally, the human CD4 gene was analyzed by Z. Hanna et al. (Mol. and Cell. Biology (1994) p. 1084-1094) in a construct which comprises at least 3 introns of a total length of 12 kb.
All the abovementioned recent studies indicate that expression of the CD4 cene is controlled in a similar manner in human and murine cells; nevertheless, the relationships between the different regulatory elements and their functions have not been elucidated.